You heat the salts to 550-600F and immerse the blade or hardware in the molten salts. It is only in there for a few seconds, so it does not ruin the temper. The change is almost instant. Use a sturdy wire hook to dip the blade or fitting in and out.
If held there longer than a minute it could soften things a bit. However, 1095 that was fully hardened will be at Rc 57-58 if held at a 600F temper for an hour. There is no real worry about temper embrittlement with niter bluing, as the soak is far too short ( and temper embrittlement is not really a knife issue).